1. Field of the Invention
The invention is related to the field of noninvasive depth profiling of skin parameters using a photoacoustic probe and in particular to depth profiling port wine stains, skin burns and melanin distributions or concentrations.
2. Description of the Prior Art
Laser treatment of port wine stain (PWS) lesions is an elective therapy for restoring normal appearance to human skin, though the success rate depends on many factors, including epidermal melanin concentration, lesion depth, and size of the blood vessels. Epidermal melanin, a broadband optical absorber, blocks laser energy and decreases fluence at the lesion. For human skin with a high concentration of melanin, skin types III or higher, epidermal temperature may rise to such levels that irreversible damage and scarring or dyspigmentation occurs.
As a preventive measure, clinicians have utilized skin cooling, such as cryogen spray cooling (CSC) to prevent epidermal damage while still treating the deeper PWS. Using CSC, a cryogen spray is directed onto the skin surface prior to and during laser irradiation and the epidermis cools nearly instantaneously. The PWS cools at some later time, preferably long after the therapeutic laser pulse has been delivered, causing irreversible thermal damage to the lesion.
Knowing the depth profile of the PWS skin, including the spatial relationship between epidermal melanin and blood vessels, the clinician may optimize CSC parameters for treatment on an individual patient basis. Thus, we have developed a photoacoustic probe for determining PWS depth quickly and noninvasively.
Burn trauma is a major cause of injury worldwide. Treatment of burns includes excision of irreversibly damaged tissue followed by skin grafting. Currently, the burn surgeon uses subjective, inexact methods for determining excision depth which may result in further damage to viable tissue. Burn trauma in the United States accounts for 3% of all injury deaths and approximately 50,000 acute hospitalizations per year. Treatment of burns includes wound excision and skin grafts. In order to optimize treatment, the burn surgeon must accurately determine the depth of thermal injury. Currently, burn depth estimates are based on appearance and sensory function, so accurate depth determination is subjective and inexact. An objective method to determine burn depth would not only provide the surgeon with a more accurate appraisal of damage, but may also allow field personnel to perform quick and accurate measurements that aid the treatment of burns.
Many methods proposed for burn depth determination simply attempt to ascertain if the injury will heal within 3 weeks, as wounds that spontaneously heal within that period usually do so without scarring or impairment. Wounds that take longer to heal require surgical intervention to prevent complications. Exact depth determination, however, would not only give an indication of the healing potential, but also aid the burn surgeon in the assessment of debridement depth, if warranted. If depth profiles of the wounds were available, the burn surgeon could accurately determine whether tissue is necrotic, reversibly damaged, or viable. Necrotic tissue must be debrided, while reversibly damaged tissue, overlying normal, viable tissue, must be allowed to heal. Debridement should occur quickly for more rapid wound closure, prevention of infection, and thus, shortened hospital stay.
The three tissue conditions noted above have contrasting optical properties, leading one to believe that an optical probing method might be useful for burn depth profiling. Unfortunately, optical signals degrade quickly in human skin owing to its highly scattering nature. Optical coherence tomography, spectroscopy, confocal imaging, fluorescence, and laser Doppler flowmetry are all dependent on preserving information contained in the optical signal, which degrades with each photon scattering event. Additionally, with the exception of polarization sensitive optically coherent tomography (PS-OCT), these methods do not give an absolute measure of burn depth, but seek to discriminate between superficial and deep burns.
The limitations of such optical methods must be considered in their implementation for diagnosis of burn injury. PS-OCT has been used to investigate burn depth. While burn depth was measured, as determined by loss of birefringence in collagen, PS-OCT is only capable of imaging the upper 1.5 mm of human skin. This depth would be insufficient to probe the entire dermis, which may be up to 5 mm deep.
While optical methods for probing burn depth will be hampered due to photon scattering by tissue, acoustic wave propagation in tissue is unaffected by such scattering. Moreover, since nearly all biological tissue has similar acoustic impedance, acoustic scattering in soft tissue is limited. Hence, an acoustic wave can travel through layered tissue with very little signal degradation. The success of conventional ultrasound relies on such a propagation environment. In fact, ultrasound has been used to study depth of burn injury. The efficacy of the ultrasound method relied on the ability to detect damage in the deep dermal capillary plexus. The result was not an exact measure of burn depth, but an estimate of whether the injury required surgical intervention or not.
Any dermatologic laser procedure must consider epidermal melanin concentration, as it is a broadband optical absorber which affects subsurface fluence, effectively limiting the amount of light reaching the dermis and targeted chromophores. An accurate method for quantifying epidermal melanin concentration would aid clinicians in determining proper light dosage for therapeutic laser procedures. While some researchers have been able to quantify epidermal melanin concentration non-invasively using visible reflectance spectroscopy (VRS), there is currently no way to determine the distribution of melanin in the epidermis.
Melanin, a broadband optical absorber, is found in the epidermis of human skin to varying degrees, determining skin color and affecting subsurface fluence of visible light after laser irradiation. Any dermatologic laser procedure using visible wavelengths must consider epidermal melanin concentration in the interpretation of diagnostic information or in dosage estimates for therapy. For example, laser therapy of port wine stain (PWS) must consider epidermal melanin concentration in order to optimize laser fluence and cryogen spray cooling parameters. Currently, epidermal melanin concentration can be estimated noninvasively by pulsed photothermal radiometry (PPTR), visible reflectance spectroscopy (VRS) and chromameter measurements. As PPTR requires analysis using inverse algorithms, determination of epidermal melanin concentration is extremely sensitive to input parameters which can give inconsistent results. VRS and chromameter measurements show repeatable measurements of epidermal melanin concentration, though they provide no depth information. Additionally, many VRS systems and chromameters utilize an integrating sphere which averages skin reflectance over a large area (e.g. >1 cm2), making local estimates of melanin concentration impossible. Photoacoustics has been used to determine optical properties of tissue and to perform imaging. It has been demonstrated how to extract absorption and scattering from analysis of photoacoustic waves induced in tissue phantoms. The prior art has used photoacoustic analysis to detect embedded absorbers in phantoms and tissue.
What is needed is an objective, accurate means to measure burn depth.